1. Field of the Invention
This invention relates to an electric power steering controller for use in automobiles.
2. Description of the Prior Art
It is known that, as shown in the following equation (1), when a voltage Va is applied to the coil of a DC motor having a brush, a counter electromotive voltage Ve is produced, a voltage obtained by subtracting the counter electromotive voltage Ve from the application voltage Va to the coil generates a coil current, thereby causing a voltage drop Vc in the coil, and, as shown in the following equation (2), the rotation speed of the motor is proportional to the counter electromotive voltage. Conversely speaking, the counter electromotive voltage is the difference between the application voltage to the coil of the motor and the voltage reduced by the coil. The voltage drop in the coil is expressed by the total of a voltage drop caused by coil resistance and a voltage drop caused by inductance which is proportional to a differential value of current as shown in the following equation (3). The frequency band at which the coil inductance appears as a voltage drop is high and can be generally expressed by the product of a coil resistance value and a coil current as shown in the following equation (3') at a steering frequency range by a car driver except a current response.
Therefore, the rotation speed of the motor can be expressed by the following four equations. EQU Vc=Va-Ve (1) EQU Ve=Ke.multidot.w (2) EQU Vc=Ra.multidot.Ia+La.multidot.(dIa/dt) (3) EQU Vc=Ra.multidot.Ia (3') EQU w=(Va-Ra.multidot.Ia)/Ke (4)
In the above equations, Ve is a counter electromotive voltage, Va an application voltage to the coil, Vc a voltage drop in the coil, Ke a counter electromotive voltage constant, w a rotation speed of the motor, Ra a coil resistance, La a coil inductance and Ia a coil current.
For example, FIG. 28 is a flow chart showing a method for detecting the rotation speed of a motor in a conventional electric power steering controller disclosed in Japanese Laid-open Patent Application No. Hei 8-175404. In FIG. 28, steps S1 to S5 are algorithm steps for detecting the rotation speed of the motor.
A description is subsequently given of the method for detecting the rotation speed of the motor in the conventional electric power steering controller.
In the conventional rotation speed detection method, a detection value of voltage applied to both terminals of the motor and a current value running through the coil are first read in step S1. In the subsequent step S2, a coil resistance equivalent value Rac and a counter electromotive voltage constant equivalent value Kec prestored in a ROM are read. In the next step S3, a rotation speed estimation value correction gain K1 is determined based on the current value running through the coil with reference to a table prestored in the ROM. In the next step S4, a motor rotation speed estimation value west is calculated based on the following equation (5). EQU west=K1.multidot.(Vt.sub.-- sns-Isns.times.Rac)/Kec (5)
wherein west is a motor rotation speed estimation value, Vt_sns is a inter-terminal voltage measurement value, Isns a coil current measurement value, Rac a coil resistance equivalent value, Kec a counter electromotive voltage constant equivalent value, and K1 a motor rotation speed estimation value correction gain.
In the subsequent step S5, the calculated motor rotation speed estimation value west is stored in a RAM. The above procedure is repeated to calculate the motor rotation speed estimation value west each time sampling is made for digital control.
The above rotation speed estimation correction gain K1 is a coefficient which is set to be "1" when the coil current is small and approximate "0" as the coil current increases and which is used to prevent the influence of an error of a coil resistance value or an error caused by temperature variations from growing along with an increase in current.
Although the inter-terminal voltage Vt of the motor is handled as the application voltage Va to the coil in the prior art described above, the actual application voltage Va to the coil is obtained by subtracting a voltage drop Vdrop between a brush and a commutator from an inter-terminal voltage Vt as shown in the following equation (6). EQU Va=Vt-Vdrop (6)
wherein Vt is an inter-terminal voltage and Vdrop is a voltage drop between the brush and the commutator.
It is known that the voltage drop Vdrop which is a function of a coil current as shown in FIG. 27 changes according to the coil current in an extremely narrow range and changes its direction according to the direction of the coil current in a normal operation range. Since this voltage drop Vdrop is not taken into account in the prior art, an almost fixed error is always produced in the motor rotation speed estimation value west. Particularly, in a low rotation speed range, the influence of the error becomes relatively large and the accuracy of the motor rotation speed estimation value west lowers. Therefore, when control is made based on this rotation speed estimation value west, if the motor rotation speed is low and the steering torque of a driver is small, for example, when driving straight, the electric power steering controller oscillates and the steering wheel vibrates by itself though the driver does not steer a vehicle.